1. Field of the Invention
The present invention relates to a reference setting system for determining a desired reference or target cornering behavior, such as a desired yaw rate or lateral acceleration, of a motor vehicle or the like.
Determination of such a reference cornering behavior is espectially important in a steering control system for steering either or both of front and rear wheels in combination with a conventional mechanical steering linkage.
2. Description of the Related Art
To facilitate understanding the present invention, a brief reference is made to an example of a steering control system disclosed in Japanese Patent Application No. 58-201287, filed by the assignee of the present application. (The applicant(s) of the present application believes that this is not a part of the prior art.) This steering control system is arranged to steer a vehicle through front or rear wheels in cooperation with a steering system including a steering gear so as to reduce a deviation of an actual stability factor determined by input steering amount, vehicle speed, and vehicle cornering behavior such as yaw rate or lateral acceleration, from a predetermined target staility factor. This system is efficacious when a vehicle is in a steady state circular motion or when a steering speed is low. However, this system is unsatisfactory when the vehicle is in a transient motion, because this system employs the target stability factor (which is constant) as a desired reference value of the vehicle behavior, and does not take the dynamics of the vehicle into account.
For example, when the steering wheel is turned rapidly, there arises a time lag of a considerable amount between an input change of an actual steered angle of the steerable road wheels caused by the angular displacement of the steering wheel, and an output change of the actual vehicle behavior such as yaw rate or lateral acceleration. Therefore, during an initial time interval after initiation of the input change, the yaw rate and lateral acceleration of the vehicle remain almost zero, and the actual stability factor which is determined by using the yaw rate or lateral acceleration as a denominator of a fraction increases to an extremely large value. As a result, the deviation of the actual stability factor from the target stability factor is increased to a corresponding extreme value, and causes undesirable hunting in the control system. This control system tends to make the vehicle behavior oscillatory and unstable, and cannot provide an accurate control especially when the steering wheel is turned rapidly.
It is possible to solve this problem by reducing the gain of the control system. In this case, however, reduction of the control gain deteriorates the response of the control system seriously.